There is abundant evidence for a urinary concentrating defect in patients with inadequate protein intake and in patients with papillary necrosis due to sickle cell anemia and analgesic nephropathy. These observations suggest that an adequate supply of urea into the renal inner medulla is necessary for maximal urinary concentration. Understanding the regulation of urea and water transport in the inner medulla could lead to improved therapy for diseases of urinary concentration. Within the inner medulla, axial gradients of osmolality, and NaCl and urea concentration are central to, and a consequence of, the operation of the countercurrent multiplier. The long-term objectives are to determine which nephron segments within the inner medulla are responsible for the establishment, maintenance, and regulation of inner medullary function; and to understand how inner medullary cells function while being exposed to extreme changes in osmolality, and NaCl and urea concentration. Using the isolated perfused tubule technique to measure urea and water transport in rat inner medullary collecting duct (IMCD) subsegments, and biochemical techniques to measure enzyme activity in microdissected nephron segments from the inner medulla, three hypotheses will be tested: I-Urea transport across the IMCD is regulated by the osmolality of the inner medullary-interstitium. II-Urea transport across the IMCD is regulated by the same factors that regulate urea production. III-Inner medullary cells osmoregulate, in part, by regulating the production of intracellular sorbitol. Sorbitol production is regulated by the enzyme aldose reductase. Specific Aim 1 will test the effect of increasing perfusate and bath osmolality by adding either NaCl or mannitol on IMCD urea permeability. Specific Aims 2-5 will test whether IMCD urea transport is regulated by the protein content of the diet, and will test the effect of glucagon, glucocorticoids, and adrenergic agents on IMCD urea and water transport. Specific Aims 6-7 will test whether the activity of aldose reductase or sorbitol dehydrogenase is regulated by hydration state of the rat or by acute changes in osmolality, ionic strength, NaCl or urea concentration, or pH.